Your password manager is still handing over your private key to a client and you're trusting your client to handle it properly. That's the problem. The password manager is not a secure solution. The password manager itself will keep your key secure but the client you hand it to may not.
Discussion
Like I said, I would use a different account for a sketchy client. You asked how other people handled this problem, and I'm telling you. No need to get snippy just because you don't like my answer.
I'm not trying to be snippy.
I thought nostr:npub1zmc6qyqdfnllhnzzxr5wpepfpnzcf8q6m3jdveflmgruqvd3qa9sjv7f60 made an excellent point. I don’t like having to copy and paste my nsec into anything, especially because it stays in the clipboard and can be pasted accidentally. That’s been a problem since the start.
I don’t copy and paste, I autocomplete, never lands in the clipboard.